Whisk the flour, sugar, cardamom, and salt in a medium bowl until combined. Add the butter and combine with your fingers, rubbing the butter into the flour until mostly incorporated but some small pieces remain. (The streusel is ready when it holds together in large clumps when squeezed—it will still be fairly dry.) Refrigerate until ready to use.

For the muffins:

Heat the oven to 375°F and arrange a rack in the middle. Coat a 12-well muffin pan with butter or line it with paper cupcake liners.

Toss the rhubarb with 1/4 cup of the sugar in a medium bowl and let the mixture sit until the rhubarb releases some juice, about 5 minutes.

Whisk together the flour, baking powder, baking soda, salt, and cardamom in a large bowl until evenly combined. Place the remaining 1/2 cup sugar, butter, egg, vanilla, and yogurt in a separate medium bowl and whisk until smooth. Add the wet ingredients to the dry and mix just until combined, about 25 to 30 strokes. Stir in the reserved rhubarb, about another 10 to 15 strokes. (The mixture will be thick.)

Place a rounded 1/4 cup of batter into each muffin well (the wells will be full) and evenly sprinkle the streusel over top. Bake until a toothpick comes out clean, about 20 to 25 minutes. Place the muffin pan on a wire rack and let the muffins cool for 15 minutes in the pan. Remove the muffins from the pan and serve warm or let cool to room temperature.